1. Field
Example embodiments relate to an image sensor, for example, to a method and apparatus for canceling a fixed pattern noise output through each column in a CMOS image sensor.
2. Description of the Related Art
A CIS (CMOS image sensor) is widely being used in various fields due to its merits such as lower voltage operation, lower power consumption, and/or integration in a standard CMOS process, compared to a CCD (charge coupled device), and expected to replace the CCD in many fields in the future. In a CIS, an analog signal output from an APS (active pixel sensor) array is converted to a digital signal. An ADC (analog-to-digital converter) of a higher resolution is used in the CIS for the conversion to a digital signal.
The CIS may be divided into a type using a single ADC and a type using a column ADC. The single ADC type uses a single ADC operating at higher speed to convert the APS analog output signals of all columns with a predetermined or given time to digital signals. While the single ADC type may reduce a chip size, it uses a larger amount of power due to its higher speed operation.
In contrast, the column ADC type may use a plurality of ADCs having a simple structure that are arranged at the respective columns. According to the column ADC type, the chip size increases while the power consumption decreases. The column ADC type may perform a correlated double sampling (CDS) to an analog output voltage that is a pixel output signal and stores a voltage according to the result thereof. Also, the column ADC type in response to a ramp signal generated by a ramp generator may compare a voltage stored during the CDS operation with a predetermined or given reference voltage to provide a comparison result signal for generation of a digital code.
As a need for a high resolution image recently increases, many manufacturers are developing a higher resolution CIS. For a higher resolution CIS, a pixel of a small size is used to improve a feature of mass production through the reduction of the size. When a smaller pixel is used, the degradation of an image quality due to pixel supply power noise may become a bigger issue.
Furthermore, because an ADC capacitor of the minimum size may be used, as the capacitance of the ADC capacitor reduces, each column of the ADC may be mismatched so that a column fixed pattern noise (CFPN) is generated. CFPN may be recognized with the human eye by increasing a system gain in a dark condition.